Autism is a pervasive neurodevelopmental disorder characterized by difficulties in social interaction and language and communication, as well as the expression of restricted, repetitive behavior. Prevalence rates of autism have increased significantly, with some estimates in the range of one in 500 individuals affected. Although autism is highly heritable (as high as 90% concordance rate for monozygotic twins), the phenotype is quite variable. Moreover, a large number of potential genetic (e.g., chromosome 7) and environmental (e.g., toxins, viruses, food constituents) factors may be involved suggesting multiple, varying etiologies.

Thus, autism is an enormously heterogeneous disorder with a wide range of symptom expression across individuals as well as across disorders on the autism spectrum (high-functioning autism, Asperger's syndrome, pervasive developmental disorder not otherwise specified). In addition, up to 75% of individuals with autism function within the mental retardation range of intellectual development, whereas individuals with high-functioning autism or Asperger's syndrome can be highly intelligent. There is also significant comorbidity in this population, with affective and attention disorders commonly observed. All of these factors make effective pharmacological treatment a challenging proposition.

To add to the challenge, no medication is available that effectively treats the core social and communication deficits that define autism. As we shall illustrate in the following sections, that is not the case for restricted, repetitive behaviors. Nonetheless, current drug therapies have been termed palliative treatments (Gerlai and Gerlai, 2004), and behavioral interventions remain the mainstay of treatment. Thus, it is critical to focus on target symptoms or behaviors commonly associated with autism (e.g., aggression, anxiety) when discussing use of psychotropic agents.

As neither genetic nor environmental causes of autism have been identified, it has been difficult to firmly establish molecular mechanisms that would serve as potential targets for pharmacological intervention. Thus, medications that have been tested for efficacy in individuals with autism have been selected on the basis of their effectiveness in relevant disorders (e.g., obsessive-compulsive disorder, anxiety) or target symptoms (aggression, hyperactivity). These efficacy tests can be characterized by the paucity of well-controlled studies of pharmacological agents for specific symptoms or target behaviors. We will highlight some of these findings by drug class, with emphasis on the few available double-blind, placebo-controlled trials.